Reading a list !

[spank]

i have the following code

typedef struct nod
{
        int inf;
        struct nod *next;
} nod;


int main(int argc,char **argv)
{
...
read(argv[1],a,b,&sgna,&sgnb);
...
}

void read(char *fis,nod *a,nod *b,int *sgna,int *sqnb)
{
        FILE *f=fopen(fis,"rt");
        int c;
        nod *t;
        if(f==NULL)
        {
                exit(-1);
        }

        a=(nod*)malloc(sizeof(nod));
        t=a;
        while((c=fgetc(f))!='\n')
        {
                if(c=='-') *sgna=1;
                else
                {
                        t->inf=c-'0';
                        t->next=(nod*)malloc(sizeof(nod));
                        t=t->next;
                }
        }
/*
        while(a->next!=NULL)
        {
                printf("%d ",a->inf);
                a=a->next;
        }
                printf("\n");
*/
        fclose(f);
}

The problem is that outside the read function the nod structure doesn't preserve its values. Inside the function it can be printed just fine.
What could the problem be ?

[dwks]

read(argv[1],a,b,&sgna,&sgnb);

You'll want

read(argv[1],&a,&b,&sgna,&sgnb);

[spank]

a and b are declared like this:

nod *a, *b;

[Dave_Sinkula]

It sounds like typical pass-by-value stuff.

#include <stdio.h>

void foo(int a)
{
   a = 42;
   printf("a = %d\n", a);
}

int main(void)
{
   int a = 1;
   printf("a = %d\n", a);
   foo(a);
   printf("a = %d\n", a);
   return 0;
}

/* my output
a = 1
a = 42
a = 1
*/

Only in your case, the parameter is a pointer that is modified in the function. Either pass a pointer to the pointer or return the pointer.

[spank]

if i:

a=b=(nod*)malloc(sizeof(nod));

in main before calling the function it works just fine!

thank you!

ps: how would the code look like if i do a pointer to a pointer ?

[Dave_Sinkula]

ps: how would the code look like if i do a pointer to a pointer ?

Uglier.

Something like this, perhaps.

void read(char *fis,nod **a,nod *b,int *sgna,int *sqnb)
{
        FILE *f=fopen(fis,"rt");
        int c;
        nod *t;
        if(f==NULL)
        {
                exit(-1);
        }

        *a=(nod*)malloc(sizeof(nod));
        t=*a;
        while((c=fgetc(f))!='\n')
        {
                if(c=='-') *sgna=1;
                else
                {
                        t->inf=c-'0';
                        t->next=(nod*)malloc(sizeof(nod));
                        t=t->next;
                }
        }
        fclose(f);
}

[quzah]

But why are you casting malloc?


Quzah.

[spank]

why not ?

is calloc better ?

[Dave_Sinkula]

FAQ > Explanations of... > Casting malloc

[spank]

so what should i use instead of malloc ?

[quzah]

Use malloc, just stop casting it. Did you even read the link?

*magic 8-ball*

"My sources say no."


Quzah.

[spank]

really sorry... there was a problem with my english. thank you for the advice!

Great forum!